This is a testharness.js-based test.
[FAIL] e.style['-webkit-mask-position-x'] = "x-start" should set the property value
  assert_not_equals: property should be set got disallowed value ""
[FAIL] e.style['-webkit-mask-position-x'] = "x-end" should set the property value
  assert_not_equals: property should be set got disallowed value ""
[FAIL] e.style['-webkit-mask-position-x'] = "0.5em, x-start, x-end" should set the property value
  assert_not_equals: property should be set got disallowed value ""
Harness: the test ran to completion.